Highly rigid propylenic resin and blow molded article made therefrom

ABSTRACT

There are provided a highly rigid propylenic resin which has a melt index MI in the range of 0.1 to 1.2 g/10 minutes as determined at 230° C. under 2.160 kg load and also satisfies a relationship between the MI and the elongational viscosity  Y(Pa·s)!, said relationship being represented by the expression 
     
         2.0×10.sup.5 ×MI.sup.-0.68 ≦Y≦8.0×10.sup.5 
    
      ×MI -0 .68 ; 
     and a blow molded article made from the above resin. The propylenic resin has favorable resistance to draw down and can produce a large-sized and lightweight blow molded article excellent in rigidity, dimensional stability and heat resistance.

TECHNICAL FIELD

The present invention relates to a highly rigid propylenic resin and ablow molded article made therefrom. More particularly, it pertains to ahighly rigid propylenic resin which has favorable resistance to drawdownand can produce a large sized and lightweight blow molded part beingexcellent in rigidity, dimensional stability and heat resistance and toa blow molded article obtained therefrom which is particularly favorablyusable for bumpers such as bumpers and bumper beams for automobiles.

BACKGROUND ART

Polypropylene as a resin for general purpose has heretofore been moldedinto a product with a desirable form and shape by any of various moldingmethods including extrusion molding, injection molding and blow molding.Of the above-mentioned molding methods, blow molding method has foundits extensive use in molding large-sized structural materials typifiedby car parts because of its advantages in that molds therefor areinexpensive and that the production process can be simplified byintegral molding methods. In this case, propylenic resin is frequentlyand extensively employed as raw materials from the viewpoint of specificgravity, rigidity, dimensional stability, heat resistance and the like.

However, the propylenic resin for general purpose is not necessarilysufficient in satisfying rigidity or resistance to drawdown which isrequired in blow molding and thus, various attemps have been made toimprove such properties. There is proposed for example, in JapanesePatent Publication No. 36609/1988 (corresponding to U.S. Pat. No.4,550,145), a process for producing propylenic resin improved indrawdown property by constituting a combination of propylene homopolymerand propylene/ethylene copolymer at a specific intrinsic viscosity and aspecific compositional ratio. However, such propylenic resin is notsufficient in rigidity when made into a molding, and thus furtherimprovement has been desired. There are also disclosed a technique ofimproving the rigidity thereof by means of multistage polymerization anda nucleating agent (refer to Japanese Patent Publication No.74264/1991), a technique of improving the resistance to drawdown thereofby means of specific multistage polymerization and a specific nucleatingagent (refer to Japanese Patent Application Laid-Open No. 213547/1988)and the like technique.

However, although these techniques improve the rigidity and resistanceto drawdown of propylenic resin to some extent, the problems stillremain unsolved in that when an attempt is made to form a so-calledlarge-sized blow molded part, that is, weighing about 5 kg or more,insufficiency in resistance to drawdown makes molding itself impossibleor makes the thickness distribution ununiform, thus resulting in failureto produce a satisfactory molded product. Such being the case, theactual state is that in forming a large-sized blow molded articleweighing 5 kg or more, propylenic resin is blended with polyethylenesuch as high density polyethylene to contrive to solve the problem ofdrawdown. Nevertheless, the blending of high density polyethylenegreatly lowers the rigidity of the blended resin, and therefore, aninorganic filler such as talc is actually added to the resin.

At any rate, the characteristics inherent in propylenic resin are lostby blending polyethylene, talc or the like and in particular, thedisadvantage that the pinch-off strength in blow molding is extremelylowered is caused thereby. Accordingly, it is desired to realize aspecific propylenic resin capable of forming a large-sized blow moldedarticle and coping with increase in weight due to the blending of talc.The above-mentioned problems should be solved in view of not only thetechnical aspect but also the social circumstances including the recycleof molded articles.

DISCLOSURE OF THE INVENTION

Under such circumstances, it is an object of the present invention todevelop a highly rigid propylenic resin which has favorable resistanceto drawdown and can produce a large sized, lightweight blow-molded partbeing excellent in rigidity, dimensional stability and heat resistance.It is another object to provide a blow molded article composed thereofwhich is favorably usable particularly for large-sized car parts such asbumpers, bumper beams, seat back and instrument panels.

In order to develop a highly rigid propylenic resin having the favorableproperties as mentioned above and a blow molded article composedthereof, intensive research and investigation were accumulated by thepresent inventors. As a result, it has been found that theabove-mentioned objects can be attained by means of a propylenic resin,especially a propylenic resin which is obtained through the formation ofa propylene polymer and a propylene/ethylene copolymer by multistagepolymerization, said resin having a melt index within a specific rangealso having a specific relationship between said melt index and theelongational viscosity thereof. The present invention has beenaccomplished by the foregoing finding and information.

That is to say, the present invention provides a highly rigid propylenicresin, especially a highly rigid propylenic resin which is obtainedthrough the formation of a propylene polymer and a propylene/ethylenecopolymer by multistage polymerization, said resin having a melt indexMI! in the range of 0.1 to 1.2 g/10 minutes as determined at atemperature of 230° C. under a load of 2,160 g and also satisfying arelationship between the MI! and the elongational viscosity Y(Pa·s)!,said relationship being represented by the expression

    2.0×10.sup.5 ×MI.sup.-0.68 ≦Y≦8.0×10.sup.5 ×MI.sup.-0.68

THE MOST PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

It is indispensable in the propylenic resin according to the presentinvention that the melt index MI! determined at a temperature of 230° C.under a load of 2,160 g in accordance with JIS K-7210, be in the rangeof 0.1 to 1.2 g/10 minutes. An MI value less than 0.1 g/10 minutesbrings about a remarkable decrease in the throughput quantity, thusdeteriorating the productivity of the resin, whereas that more than 1.2g/10 minutes makes it impossible to form a large-sized blow moldedarticle. Taking moldability etc. into consideration, the MI ispreferably in the range of 0.2 to 1.0 g/10 minutes.

In addition, the propylenic resin according to the present inventionsatisfies a relationship between the MI and the elongational viscosityY(Pa·s)!, said relationship being represented by the expression

    2.0×10.sup.5 ×MI.sup.-0.68 ≦Y≦8.0×10.sup.5 ×MI.sup.-0.68,

preferably

    2.3×10.sup.5 ×MI.sup.-0.68 ≦Y≦4.8×10.sup.5 ×MI.sup.-0.68,

a Y value less than 2.0×10⁵ ×MI⁻⁰.68 results in severe drawdown of aparison at the time of blow molding, making it difficult to form alarge-sized blow molded article weighing 5 kg or more, whereas that morethan 8.0×10⁵ ×MI⁻⁰.68 leads to deterioration of the extrusioncharacteristics as well as the external appearance of the blow moldedarticle.

The elongational viscosity Y(Pa·s)! is measured with a stretch rheometer(for example, produced by Iwamoto Manufacturing Co., Ltd.) by the use ofa bar sample with c.a. 3 mm diameter and 20 cm length by allowing thesample to stand in a silicone oil at 175° C. for 15 minutes under theconditions including 175° C. temperature, 0.05 sec ⁻¹ strain velocityand 2.0 strain.

The process for producing the propylenic resin according to the presentinvention is not specifically limited provided that the process iscapable of producing a propylenic resin which satisfies theabove-mentioned conditions. There are usable a variety of processes, ofwhich is preferable a process for producing a propylene polymer and apropylene/ethylene copolymer by multistage polymerization.

As the favorable multistage polymerization method, mention is made of aprocess in which through the use of a stereoregular catalyst, propylenepolymers each having a different intrinsic viscosity η! from one anotherare produced in the first and second stages, and a propylene/ethylenecopolymer is produced in the third stage.

Examples of the aforesaid stereoregular catalyst to be used for themultistage polymerization include a catalyst comprising a halogenide ofa transition metal, an organoaluminum compound and a substance to beadded for preparing a polymer having improved stereoregularity and abroad molecular weight distribution such as a lactone.

Examples of the halogenide of a transition metal preferably includehalogenides of titanium, of which titanium trichloride is particularlypreferable. The titanium trichloride is exemplified by that prepared byreducing titanium tetrachloride through any of various methods; thatprepared by further activating the preceding titanium tetrachloridethrough any of various methods; that prepared by further activating thepreceding titanium trichloride by means of a treatment in a ball milland/or solvent cleaning (for example, cleaning with an inert solvent oran inert solvent containing a polar compound); and that prepared bysubjecting titanium trichloride or a titanium trichloride eutectic suchas TiCl₃.1/3 AlCl₃ to a crushing treatment together with an amine, anether, an ester, sulfur, a halogen derivative or an organic or inorganicnitrogen or phosphorus compound. There is also usable a halogenide oftitanium supported on a magnesium-based carrier.

Examples of the organoaluminum compound include a compound representedby the general formula (I)

    AlR.sub.n X.sub.3-n                                        (I)

wherein R is an alkyl group having 1 to 10 carbon atoms, X is a halogenatom, and n is a number satisfying 0<n≦3.

Specific examples of such organoaluminum compound includedimethylaluminum chloride, diethylaluminum chloride, ethylaluminumsesquichloride, ethylaluminum dichloride and triethylaluminum. Theorganoaluminum compound may be used alone or in combination with atleast one other. It is used in an amount of usually 1 to 100 moles perone mole of the above-mentioned halogenide of a transition metal.

Example of lactones include a compound represented by the generalformula (II) ##STR1## wherein R¹ and R² are each a hydrogen atom or ahydrocarbon group having at most 20 carbon atoms and belonging tosaturated aliphatic series, unsaturated aliphatic series, alicylicseries or aromatic series, and may be the same as or different from eachother, and m is an integer from 2 to 8.

As the lactone of the general formula (II), mention is made of γ-lactonssuch as γ-butyroloctone, γ-valerolactone, γ-caproloctone,γ-capryloctone, γ-laurolactone, γ-palmilactone, γ-stearolacton;δ-lactones such as δ-valerolactone and δ-caproloctone; ε-lactons such asε-caprolactone; and β-loctones such as β-propiolactone anddimethylpropiolactone. Of these lactones, γ-loctones and ε-lactones arepreferable, and γ-butyrolactone, γ-caprolactone and ε-caproloctone areparticularly preferable. Any of these lactones may be used alone or incombination with at least one other. It is used in an amount of usually0.01 to 10 moles per one mole of the above-mentioned halogenide of atransition metal.

In the foregoing multistage polymerization, it is preferable in thefirst stage to carry out the polymerization of propylene at atemperature of 50° to 70° C. so as to produce a propylene polymer havingan intrinsic viscosity η! of 0.5 to 3.5 dL (deciliter)/g (in decalin,135° C.) in an proportion of 60 to 80% by weight based on the wholeamount of the polymer. An intrinsic viscosity η! of the propylenepolymer less than 0.5 dL/g brings about a low impact strength of thepropylenic resin to be produced, whereas that more than 3.5 dL/g causesa decrease in throughput quantity of the resin at the time of blowmolding, in certain cases. A proportion of the polymer produced in thefirst stage less than 60% by weight results in insufficient rigidity ofthe propylenic resin to be produced, whereas that more than 80% byweight gives rise to deterioration of the impact strength thereof, incertain cases.

Next, it is preferable in the second stage to carry out thepolymerization of propylene at a temperature of 50° to 70° C. so as toproduce a propylene polymer having an intrinsic viscosity η! of 3.5 to5.5 dL/g (in decalin, 135° C.) in a proportion of 10 to 20% by weightbased on the whole amount of the polymer. An intrinsic viscosity η! ofthe propylene polymer less than 3.5 dL/g brings about a low impactstrength of the propylenic resin to be produced, whereas that more than5.5 dL/g causes a decrease in throughput quantity of the resin at thetime of blow molding, in certain cases. A proportion of the polymerproduced in the second stage less than 10% by weight results ininsufficient rigidity of the propylenic resin to be produced, whereasthat more than 20% by weight gives rise to deterioration of the impactstrength thereof, in certain cases.

Moreover, it is preferable in the third step to carry out thecopolymerization of propylene and ethylene at a temperature of 45° to65° C. so as to produce a propylene/ethylene copolymer having anintrinsic viscosity η! of 3.5 to 5.5 dL/g (in decalin, 135° C.) and anethylene unit content of 40 to 75% by weight in a proportion of 8 to 15%by weight based on the total amount of the polymer. An intrinsicviscosity η! of the propylene/ethylene copolymer of less than 3.5 dL/gbrings about a low impact strength of the propylenic resin to beproduced, whereas that more than 5.5 dL/g causes a decrease inthroughput quantity of the resin at the time of blow molding, in certaincases. A proportion of the copolymer produced in the third stage lessthan 8% by weight results in a low impact strength of the propylenicresin to be produced, whereas that more than 15% by weight gives rise todeterioration of the rigidity thereof, in certain cases. An ethyleneunit content of less than 40% by weight in the copolymer results in alow impact strength of the propylenic resin to be produced, whereas thatof more than 60% by weight leads to deterioration of the rigidity, incertain cases. The ethylene unit content in the copolymer can beobtained by measuring infrared absorption spectrum.

The modulation for the intrinsic viscosity η! of the polymer in each ofthe stages can be carried out, for example, by properly altering theconcentration of a molecular weight modulator such as hydrogen. Thepressure in the polymerization reaction is selected in each stage in therange of usually atmospheric pressure to 30 kg/cm² G, preferably 1 to 15kg/cm² G.

As the polymerization system, there are applicable a continuous methodby using at least three polymerization vessels, a batchwise method byusing at least one polymerization vessel and a method by the combinationof the above-mentioned continuous method and batchwise method. As thepolymerization method, there are adoptable, without specific limitation,suspension polymerization, solution polymerization, gas-phasepolymerization or the like.

As a solvent, when used, mention is made of an aliphatic hydrocarbonsuch as heptane and hexane, an alicyclic hydrocarbon such as cyclohexaneand an aromatic hydrocarbon such as benzene and toluene. Any of thesolvents may be used alone or in combination with at least one other.

The propylenic resin thus obtained according to the present inventionhas favorable resistance to drawdown and can afford a large-sized blowmolded part which is lightweight and excellent in rigidity, dimensionalstability and heat resistance.

The blow molded article according to the present invention is producedthrough the blow molding of the above-mentioned propylenic resin byblending therewith as desired, any of additives such as soft elastomer,modified polyolefin, antioxidant, heat resistant stabilizer, weatherresistance stabilizer, inorganic or organic filler, nucleating agent,antistatic agent, chlorine scavenger, slip agent, flame retardant andcoloring agent. As the blow molding method, there is usable, withoutspecific limitation, a method which has been customarily employed in theblow molding of propylenic resin.

In comparison with the blow molded article produced by blow molding thepolypropylene blended with a large amount of an inorganic filler such astalc which has heretofore been used in general, the blow molded articleaccording to the present invention is lightweight and excellent inrigidity, dimensional stability and heat resistance, and is preferablyused particularly for bumpers including car bumpers and car bumperbeams.

In the following, the present invention will be described in more detailwith reference to examples, which however, shall not be construed tolimit the present invention thereto.

Determinations were made of the melt index MI ! and elongationalviscosity Y ! of the propylenic resin by the methods described herein,of the ethylene unit content by measuring the infrared absorptionspectrum, of the tensile modulus according to JIS K7113, and of the Izodimpact value (at -20° C.) according to JIS K7110.

The intrinsic viscosity η! of the polymer in each of the stages is thatmeasured in decalin at 135° C.

EXAMPLE 1

A 10 L (liter) autoclave equipped with a stirrer was charged with 4L ofn-heptane, 5.7 mmol of diethylaluminum chloride, 0.7 g of titaniumtrichloride and 0.2 mL (milliliter) of ε-caprolactone.

Thereafter, the autoclave was continuously fed with hydrogen which hadbeen weighed so as to attain a prescribed intrinsic viscosity η! of thepropylene polymer to be produced and with propylene so as to attain areaction pressure of 9 kg/cm² G, while the liquid-phase temperature wasmaintained at 60° C., to carry out the first stage reaction understirring for 90 minutes. Subsequently, the unreacted propylene wasremoved, and hydrogen thus weighed along with propylene werecontinuously fed in the autoclave so as to attain a reaction pressure of7 kg/cm² G, while the temperature therein was maintained at 60° C., tocarry out the second stage reaction for 40 minutes.

Further, the mixture of propylene and ethylene and hydrogen thus weighedwere continuously fed in the autoclave so as to attain a reactionpressure of 5 kg/cm² G, while the temperature therein was maintained at57° C., to carry out the third stage reaction for 30 minutes.

To the resultant polymerization product was added n-butanol, and themixture was stirred at 65° C. for one hour to decompose the catalyst andwas subjected to the steps of separation, cleaning and drying with theresult that propylenic resin in the form of white powder was obtained.

The intrinsic viscosity η! and the polymerization amount of the polymerobtained in each of the polymerization stages are given in Table 1.Further, the physical properties of the objective propylenic resin aregiven in Table 2

EXAMPLE 2

The procedure in Example 1 was repeated to carry out the polymerizationexcept that the intrinsic viscosity η! and the polymerization amount ofthe polymer obtained in each of the polymerization stages were alteredas shown in Table 1. The results obtained are given in Table 2.

EXAMPLE 3

The procedure in Example 1 was repeated to carry out the polymerizationexcept that the intrinsic viscosity η! and the polymerizaiton amount ofthe polymer otained in each of the polymerization stages were altered asshown in Table 1. The results obtained are given in Table 2.

EXAMPLE 4

To the polymer which had been obtained in Example 1 was added 0.1% byweight of sodium salt of methylenebis(2,4-di-tert-butylphenol) acidphosphate as the nucleating agent to form a propylenic resin. Thephysical properties thereof are given in Table 2.

COMPARATIVE EXAMPLES 1 & 2

A 10 L(liter) autoclave equipped with a stirrer was charged with 5 L ofdehydrated n-hexane, 1.0 g of diethylaluminum chloride and 0.3 g oftitanium trichloride.

Thereafter, the autoclave was continuously fed with hydrogen which hadbeen weighed so as to attain a prescribed intrinsic viscosity η! of thepropylene polymer to be produced and with propylene so as to attain areaction pressure of 9 kg/cm² G, while the liquid-phase temperature wasmaintained at 65° C., to carry out the first stage reaction understirring for 90 minutes. Subsequently, the unreacted propylene wasremoved, and the liquid-phase temperature was lowered to 50° C.

Then, hydrogen thus weighed along with propylene were continuously fedin the autoclave so as to maintain a reaction pressure of 9 kg/cm² G,and a reaction temperature of 50° C., to carry out the second stagereaction for 40 minutes.

Further, the mixture of propylene and ethylene and hydrogen thus weighedwere continuously fed in the autoclave so as to attain a reactionpressure of 9 kg/cm² G, while the temperature therein was maintained at50° C., to carry out the third stage reaction for 30 minutes. Then, theunreacted gas was removed, and to the resultant polymerization productwas added n-butanol, and the mixture was stirred at 65° C. for one hourto decompose the catalyst and was subjected to the steps of separation,cleaning and drying with result that a polymer in the form of whitepowder was obtained.

The polymerization was carried out by altering the intrinsic viscosityη! and the polymerization amount of the polymer in each of thepolymerization stages as given in Table 1. Further, the physicalproperties of the objective polypropylenic resin are given in Table 2.

COMPARATIVE EXAMPLE 3

The procedure in Example 1 was repeated to carry out the polymerizationexcept that the addition of ε-caprolactone as the catalyst was omitted.The intrinsic viscosity η! and the polymerization amount of the polymerin each of the polymerization stages are given in Table 1, and thephysical properties of the objective propylenic resin are given in Table2.

COMPARATIVE EXAMPLE 4

The procedure in Example 1 was repeated to carry out the polymerizationexcept that the intrinsic viscosity η! and the polymerization amount ofthe polymer obtained in each of the polymerization stages were alteredas shown in Table 1. The results obtained are given in Table 2.

COMPARATIVE EXAMPLE 5

The procedure in Example 1 was repeated to carry out the polymerizationexcept that the polymerization was performed in two stages instead ofthree stages and instead, the polymerization amount in the first stagewas increased. The instrinsic viscosity η! and the polymerization amountof the polymer in each of the polymerization stages are given in Table1, and the physical properties of the objective propylenic resin aregiven in Table 2.

COMPARATIVE EXAMPLE 6

The mixture having the under-mentioned composition was incorporated witha prescribed antioxidant and thereafter was kneaded by the use of abidirectional twin-screw kneader (produced by Kobe Steel Ltd., model2FCM) at a temperature set to 200° C. at a number of screw revolutionsof 800 r.p.m, while the temperature of the resulting melt was 250° C.The kneaded product was formed into strands by means of an extruder, andthen granulated with a pelletizer to produce a composite material forbumper beam. The results of the measurement of the physical propertiesare given in Table 2.

Polypropylene (ethylene unit content: 5% by weight, MI: 0.9 g/10minutes) 70% by weight

High density polyethylene (HLMI: 3.8 g/10 minutes) 20% by weight

Talc 10% by weight

where HLMI indicates MI (melt index) measured under the conditions of190° C. temperature and 21.6 kg load.

                  TABLE 1                                                         ______________________________________                                        First stage    Second stage Third stage                                               polymerization   polymerization                                                                             polymerization                                  amount           amount       amount                                   η! (% by weight)                                                                             η!                                                                             (% by weight)                                                                           η!                                                                           (% by weight)                           ______________________________________                                        Exam- 3.0   74         4.5 14       4.5 12                                    ple 1                                                                         Exam- 2.4   74         4.5 12       4.8 14                                    ple 2                                                                         Exam- 2.4   75         4.0 16       4.2  9                                    ple 3                                                                         Exam- 3.0   74         4.5 14       4.5 12                                    ple 4                                                                         Comp. 2.9   76         5.9 10       4.6 14                                    Exam-                                                                         ple 1                                                                         Comp. 2.4   78         7.2 12       9.4 10                                    Exam-                                                                         ple 2                                                                         Comp. 3.0   75         4.3 13       4.5 12                                    Exam-                                                                         ple 3                                                                         Comp. 1.5   78         4.8 12       5.3 10                                    Exam-                                                                         ple 4                                                                         Comp. 3.4   89         --  --       4.2 11                                    Exam-                                                                         ple 5                                                                         ______________________________________                                         Remarks: Comp. = Comparative                                             

                  TABLE 2-1                                                       ______________________________________                                                 Physical properties of propylenic resin                                       ethylene unit content                                                                     MI                                                                (% by weight)                                                                             (g/10 minutes)                                           ______________________________________                                        Example 1  6.0           0.30                                                 Example 2  6.3           0.60                                                 Example 3  6.3           0.80                                                 Example 4  6.0           0.30                                                 Comp.      2.9           0.27                                                 Example 1                                                                     Comp.      6.2           0.36                                                 Example 2                                                                     Comp.      6.0           0.30                                                 Example 3                                                                     Comp.      4.7           8.50                                                 Example 4                                                                     Comp.      5.3           0.30                                                 Example 5                                                                     ______________________________________                                    

                  TABLE 2-2                                                       ______________________________________                                        Physical properties of propylenic resin                                                        Ixod impact value                                                                         elongational                                     Tensile modulus  at -20° C.                                                                         viscosity Y                                      (MPa)            (kJ/m2 )    (Pa · s)                                ______________________________________                                        Example 1                                                                            1,490         3.7         6.5 × 10.sup.5                         Example 2                                                                            1,400         4.4         4.2 × 10.sup.5                         Example 3                                                                            1,520         3.2         2.8 × 10.sup.5                         Example 4                                                                            1,650         3.9         6.8 × 10.sup.5                         Comp   1,600         3.0         4.2 × 10.sup.5                         Example 1                                                                     Comp   1,500         3.5         2.8 × 10.sup.5                         Example 2                                                                     Comp.  1,380         3.6         4.0 × 10.sup.5                         Example 3                                                                     Comp.  1,580         3.4         not                                          Example 4                        measurable                                   Comp.  1,450         3.9         3.1 × 10.sup.5                         Example 5                                                                     Comp.  1,500         3.5         2.8 × 10.sup.5                         Example 6                                                                     ______________________________________                                    

EXAMPLES 5 to 8 and COMPARATIVE EXAMPLES 7 to 12

Each of the propylenic resins obtained by means of scale up and inaccordance with the conditions in Examples 1 to 4 and ComparativeExamples 1 to 6, respectively, was molded into a car bumper beam(1400×100×100 mm in size and 5 kg in weight) and a truck bumper(2100×400×70 mm in size and 7.2 kg in weight) under the moldingconditions and temperature conditions as desribed hereunder, except thatthe truck bumper was produced only in Example 8.

    ______________________________________                                        (Molding conditions)                                                          molding machine:   90 mm in diameter                                          screw:             90 mm in diameter                                          die:               100 mm in diameter                                         accumulator:       15 liter (car bumper beam)                                                    25 liter (truck bumper)                                    mold clamping force:                                                                             60 ton                                                     number of screw revolutions:                                                                     40 r.p.m.                                                  electric motor load:                                                                             115 A                                                      (Temperature conditions)                                                      cylinder                                                                      No. 1:             230° C.                                             No. 2:             210° C.                                             No. 3:             190° C.                                             No. 4:             190° C.                                             crosshead                                                                     No. 1:             190° C.                                             No. 2:             190° C.                                             No. 3:             190° C.                                             die                                                                           No. 1:             190° C.                                             No. 2:             190° C.                                             molding cycle:     200 sec                                                    mold temperature:  28° C.                                              resin temperature: 225° C.                                             ______________________________________                                    

Investigations were made of (1) moldability, (2) thickness distributionand appearance, (3) product rigidity, (4) throughput quantity, (5)pinch-off strength and (6) impact resistance for each of the car bumperbeams and truck bumpers produced in the aforesaid manners, and overallevaluations were carried out based on the investigation results. Theevaluation results are given in Tables 3 to 5, in which Table 3 givesthe results obtained in Example 8 only, while Tables 4 and 5 give theresults obtained in the production of car bumper beams.

The measurement and evaluation in each of the investigation items werecarried out according to the following standards.

(1) Moldability

Propylenic resin for a bumper beam parison with necessary length/weightof 1900 mm/10 kg and that for a bumper parison with necessarylength/weight of 2600 mm/15 kg were each injected from an accumulator toform a parison. The moldability was evaluated by the variation in thelength of the parison during 5 seconds, that is, the mold closing time.

    ______________________________________                                        L/Lo < 1.10            ⊚ Good                                  1.10 ≦ L/Lo ≦ 1.15                                                                     ∘ Fair                                     L/Lo > 1.15            x Poor                                                 ______________________________________                                    

where, Lo: parison length at the end of injection

L: parison length after 5 seconds from the end of injection

(2) Thickness distribution

Thickness distribution was evaluated by measuring the thickness of eachof the cross sections of the blow molded article.

    ______________________________________                                        Variation in thickness ≦ 10%                                                                    ⊚ Good                                Variation in thickness > 10%, ≦ 20%                                                             ∘ Fair                                   Variation in thickness > 20%                                                                           x Poor                                               ______________________________________                                    

(3) Product rigidity (rigidity for dishing at 100 kg)

Product rigidity was evaluated by comparing its distorsion with that ofa steel-made product.

    ______________________________________                                        ∘ ≦ 3 mm in distortion                                                     (same as or smaller than the                                                  distortion of the steel-made)                                 x > 3 mm in distortion                                                                        (larger than the distortion                                                   of the steel-made)                                            ______________________________________                                    

(4) throughput performance

Throughput performance was evaluated by measuring the throughputquantity per one hour by the use of a blow molding machine of 90 mm indiameter, and by comparing the throughput quantity thus measured withthat of the composite material used in Comparative Example 6.

⊚ Superior to the composite material

◯ Comparable to the composite material

(5) Pinch-off strength

In the production of a blow molded article, a fusing adhesion part whichis called pinch-off part is formed at the time of mold closing by thefusing adhesion of the inside of a parison with each other. Thepinch-off part is apt to become a starting point of rupture or break,and thus a structural part and a part requiring strength are needed tobe improved in the fusing adhesivity of the pinch-off part. Accordingly,the fusing adhesivity of the pinch-off part was evaluated by thefollowing procedure.

By the use of each of the resins that had been obtained in Example 4 andComparative Examples 6, a bottle having a prescribed shape was preparedby blow molding, and a strip test piece with 20 mm width was cut outfrom the bottom of the bottle so as to include the pinch-off in thewidth direction. The test piece thus obtained was notched with anotching blade of 2.0 mm in R at both the ends of the pinch-off part sothat the pinch-off part had 10^(mm) width. The test piece was tested ata tensile velocity of 50 mm/minute by the use of a tensile test machine(produced by INSTRON Corp. in U.S.A under the tradename INSTRON 1125).The yield strength and rupture energy were regarded as the index of thepinch-off strength. Specifically, the enhancement of the fusingadhesivity increases with an increase in each of the foregoing values.The yield strength is represented by the maximum stress value in thestress strain diagram, while the rupture energy is represented by thethe product (stress)×(strain)! which is obtained by integrating thestress with respect to the strain from zero to the rupture point in therange of the strain. For the sake of simplicity, the rupture energy canbe shown by the area which is surrounded by the stress strain diagramand the abscissa.

(6) Impact resistance

A bumper beam which had been obtained by blow molding each of the resinsas obtined in Example 4 and Comparative Example 6 was subjected topendulum test in accordance with Federal Motor Vehicle Safety Standards(abbreviated to FMVSS) PART 581. Specifically, the bumper beam wasfitted to a bogie of 1000 kg in weight, and an impact ridge of 1000 kgin weight was allowed to collide with the bumper beam at a velocity of 5miles/hour (about 8 km/hour) to obtain the relation between thegenerated load and the distortion size of the beam, while the place ofthe collision was made to the central part of the beam. The testtemperatures were each set to a high temperature (50° C.), ordinarytemperature (23° C.) and a low temperature (-10° C. and -30° C.) takinginto consideration of the conditions under which the bumper beam ismounted on a commercial car. The evaluation was carried out by themaximum distortion and the occurrence of crack.

                  TABLE 3                                                         ______________________________________                                                  Cap bumper beam                                                                           Truck bumper                                            ______________________________________                                        Parison length                                                                            1,900 mm      2,600 mm                                            Parison weight                                                                              10 kg         15 kg                                             Product length                                                                            1,400 mm      2,100 mm                                            Product weight                                                                               5 kg        7.2 kg                                             Moldability  1.06          1.09                                               Thickness   Thickness     Thickness                                           distribution                                                                              variation < 10%                                                                             variation < 10%                                     Rigidity    Distortion: little                                                                          Distortion: little                                  Impact      Abnormality: none                                                                           Abnormality: none                                   resistance                                                                    Overall     good          good                                                evaluation                                                                    ______________________________________                                    

                  TABLE 4-1                                                       ______________________________________                                                                Thickness                                             Resin        Moldability                                                                              distribution                                                                            External                                    used         L/Lo       (%)       appearance*.sup.1                           ______________________________________                                        Example 5                                                                             Example  1.07        7      ∘                                     1        ⊚                                                                         ⊚                                  Example 6                                                                             Example  1.10       15      ∘                                     2        ∘                                                                            ∘                                     Example 7                                                                             Example  1.15       18      ∘                                     3        ∘                                                                            ∘                                     Example 8                                                                             Example  1.06        6      ∘                                     4        ⊚                                                                         ⊚                                  Comp.   Comp.    1.15       20      ∘                             Example 7                                                                             Example 1                                                                              ∘                                                                            Δ                                           Comp.   Comp.    1.20       22      ∘                             Example 8                                                                             Example 2                                                                              x          x                                                 Comp.   Comp.    1.17       22      ∘                             Example 9                                                                             Example 3                                                                              ∘                                                                            x                                                 Comp.   Comp.    --         --      --                                        Example 10                                                                            Example 4                                                                              x          xx                                                Comp.   Comp.    1.20       25      Δ                                   Example 11                                                                            Example 5                                                                              x          x                                                 Comp.   Comp.    1.14       20      ∘                             Example 12                                                                            Example 6                                                                              ∘                                                                            ∘                                     ______________________________________                                         *.sup.1 External appearance                                                   ∘: Wrinkle or stain being hardly observed                         Δ: Wrinkle or strain being caused to some extent                   

                  TABLE 4-2                                                       ______________________________________                                                Product   throughput                                                                              Overall*.sup.2                                            rigidity  performance                                                                             evaluation                                        ______________________________________                                        Example 5 ∘                                                                             ∘                                                                           ⊚                              Example 6 ∘                                                                             ⊚                                                                        ∘                                 Example 7 ∘                                                                             ⊚                                                                        ∘                                 Example 8 ∘                                                                             ∘                                                                           ⊚                              Comp.     ∘                                                                             ∘                                                                           Δ                                       Example 7                                                                     Comp.     ∘∘                                                                x                                                       Example 8                                                                     Comp.     x           ∘                                                                           Δ                                       Example 9                                                                     Comp.     --          ⊚                                                                        x                                             Example 10                                                                    Comp.     ∘                                                                             ∘                                                                           x                                             Example 11                                                                    Comp.     ∘                                                                             ∘                                                                           Δ                                       Example 12                                                                    ______________________________________                                         *.sup.2 Overall evaluation                                                    ⊚ : Fully satisfying product performance requirement           ∘ : Satisfying product performance requirement                    Δ: Somehwat inferior to aimed product performance                       x: Greatly inferior to aimed product performance                         

                  TABLE 5                                                         ______________________________________                                                                Comparative                                                            Example 8                                                                            Example 12                                            ______________________________________                                        Pinch- Yield strength  72       43                                            off    (kgf)                                                                  strength                                                                             Rupture         94       9.0                                                  energy                                                                        (kgf · mm)                                                    Pendulum                                                                             high      maximum   55     73                                          test   temperature                                                                             distortion                                                          (50° C.)                                                                         (mm)                                                                          crack     --     --                                                 ordinary  maximum   55     73                                                 temperature                                                                             distortion                                                          (23° C.)                                                                         (mm)                                                                          carck     --     --                                                 low       maximum   38     37                                                 temperature                                                                             distortion                                                          (-10° C.)                                                                        (mm)                                                                          crack     --     --                                                 (-30° C.)                                                                        maximum   36     not                                                          distortion       measurable                                                   (mm)                                                                          crack     --     Remarkably                                                                    damaged from                                                                  pinch-off fusion                                                              part as starting                                                              point                                       ______________________________________                                    

In the overall evaluation in Table 4, the results of pinch-off strengthand impact resistance were taken into consideration as well.

In addition, it has been demonstrated in Table 5 that the yield strengthin Example 8 is about 1.7 times that in Comparative Example 12 and thefusing adhesivity in Example 8 expressed in terms of rupture energy isabout 10 times that in Comparative Example 12. It is thought that themanifestation of the excellent impact resistant in the pendulum test inExample 8 is due to the remarkable improvement in the fusing adhesivityof the pinch-off part.

INDUSTRIAL APPLICABILITY

The blow molded articles obtained from the highly rigid propylenic resinaccording to the present invention are favorably usable particularly forlarge-sized car parts such as bumpers, bumper beams, seat back andinstrument panels.

We claim:
 1. A blow molded article which comprises a rigid propylenicresin, wherein said resin has a melt index (MI) from 0.1 to 1.2 g/10minutes determined at a temperature of 230° C. and a load of 2,160 g andsatisfying a relationship between said melt index and the elongationalviscosity (Y (Pa·s)), wherein said relationship is represented by theequation:

    2.0×10.sup.5 ×MI.sup.-0.68 ≦Y≦8.0×10.sup.5 ×MI.sup.-0.68.


2. The blow molded article of claim 1, wherein said melt index is from0.2 to 1.0 g/10 minutes.
 3. The blow molded article of claim 1, whereinsaid relationship is represented by the equation:

    2.3×10.sup.5 ×MI.sup.-0.68 ≦Y≦4.8×10.sup.5 ×MI.sup.-0.68.


4. A blow molded article according to claim 1, wherein said article is acar bumper.
 5. A blow molded article according to claim 1, wherein saidarticle is a car bumper beam.
 6. A blow molded article according toclaim 1, wherein said article is a car seat back.
 7. A blow moldedarticle according to claim 1, wherein said article is a car instrumentpanel.
 8. A process of preparing a blow molded article which comprisesblow molding a rigid propylenic resin, wherein said resin has a meltindex (MI) from 0.1 to 1.2 g/10 minutes determined at a temperature of230° C. and a load of 2,160 g and satisfying a relationship between saidmelt index and the elongational viscosity (Y (Pa·s)), wherein saidrelationship is represented by the equation:

    2.0×10.sup.5 ×MI.sup.-0.68 ≦Y≦8.0×10.sup.5 ×MI.sup.-0.68.